du command, at a command-line, will tell you which directories/folders are occupying the most space. You can infer from this where the biggest files are.
$ du -h /
The -h tells du to list the sizes in 'human readable terms: kb, Mb, Gb, and so on.
If you try this as an ordinary user, you'll see error messages about directories you don't have permission to examine.
If you do it as root, there should be no error messages.
# du -h /
pipe the output through
sort -n so that the last lines show the biggest directories (or reverse sort and use head).
# du -h | sort -n
Another approach would be to use
find with options which specify 'find files bigger than a certain size' but this method requires some practice to get it right.
# find / -size +10M
will list files bigger than 10Mbyte